The backbone infrastructure on which “information technology” is running, comprises two major parts, “information processing” and “information transfer”. The information processing is done in electrical domain by exploiting ubiquitous transistors (subject of Shockley-Bardeen-Brattain's 1956 physics Nobel prize) which are integrated in quantities of millions to tens of billions in tiny integrated circuits (subject of Jack Kilby's 2000 physics Nobel prize). The frequencies at which these circuits are operating are normally in the range of tens of hertz (Hz) to tens of giga-hertz (GHz).
However, when it comes to the transfer of information, electrical signals exhibit major shortcomings, whereas signals at optical frequencies, i.e., frequencies in the range of hundreds of tera-hertz (THz), have significant advantages.
First, the carrier frequency of optical signals, at hundreds of THz, is far away from the peak of black body radiation curve, and as such thermal noise is almost nonexistent. Second, unlike the process of guiding electrical signals through metallic structures which is inherently lossy, the process of guiding optical signals (light) through dielectric structures is inherently lossless, and the loss observed in practice is mostly the result of manufacturing imperfections rather than any fundamental physical phenomena. This fact was the basis of optical communications which eventually led to Charles Kao's 2009 physics Nobel prize. Optical signals are transferred through dielectric waveguides that have losses in the order of fraction of decibels per kilometer (dB/km), compared to tens of dB per meter (dB/m) for metallic based transmission lines used for transferring radio frequency (RF) signals.
However, to take advantage of all unique features that optical communications offers, the electrical signals that perform “information processing” have to be somehow transferred to the optical domain. This is the subject of the field of optical modulation, which studies imparting signals at much lower frequencies to optical frequencies, to take advantage of all the good things at these extremely high frequencies offer.
To perform such modulation, certain propagation property of the optical signal is generally modified in a controllable fashion. This propagation property of the optical signal can be at least one of the following: phase, amplitude, spatial profile, polarization, chirp, phase velocity, group velocity, direction of propagation, oscillation frequency, frequency spectrum, wave vector, magnitudes of the three components of the electric field, magnitudes of the three components of the magnetic field, phases of the three components of the electric field, and phases of the three components of the magnetic field. To control the propagation property of the optical signal, an element with a controllable optical property is needed. The controllable optical property can be at least one of the following: effective index, absorption coefficient, group index, birefringence, index ellipsoid, and spatial distribution of refractive index. The apparatus through which, a propagation property of an optical signal is controlled using an input signal that adjusts the controllable optical property of an element is called an “optical modulator”. If the input signal is an electrical one, it can also be called an “electro-optic modulator”, an “optoelectronic modulator”, or similar names understood by those skilled in the art. If the input signal is an acoustic signal, it can be called an “acousto-optic modulator”.
This electrical signal can be either freely propagating, guided, or a combination thereof. For instance, a Pockels cell used in widely used titanium:sapphire (Ti:Sapph) laser amplifiers is generally a polarization modulator that upon application of proper electrical signal modifies the birefringence of a crystal, and through which the polarization state of the freely propagating beam of light. On the other hand, a telecommunication (telecom) grade lithium niobate (LiNbO3) modulator comprises phase modulating sections that modify the phase of a two-dimensional guided wave optical signal through ion-exchanged formed channel waveguides in the LiNbO3.
Alternatively, phase modulating a light signal traveling through a thin film based slab polymer waveguide is a combination of free space propagation in the plane of the slab, and one-dimensional wave-guiding in the direction orthogonal to the plane of the slab through the phenomenon of total internal reflection (TIR).
The major performance metrics of modulators are speed, cost, energy efficiency, and the quality of the modulated signal as quantified in multiple ways such as extinction ratio.
When designing optical modulators, multiple parameters affect the performance metrics, where major ones are the spatial profile of the optical signal, the structure of electrical port(s), the structure of the optical port(s), the nature of interaction between optical and electrical signals, and the relative position of the electrical port(s) with respect to the spatial profile of the optical signal.
As an example, in a lithium niobate modulator the nature of interaction between light and electricity is field based. In other words, the applied electrical signals generate electric fields that pass through the LiNbO3 lattice. This electric field polarizes constituent atoms leading to modification of the index ellipsoid of the material through a process, commonly known as “Pockels effect” or “linear electro-optic effect”. Since this whole phenomenon is done by virtue of fields, the immediate conclusion is that the spatial profile of the optical field can be optimized rather independently from the structure of the electrical structures that generate the field. Consequently, in a telecom grade LiNbO3 modulator, operating at tens of GHz frequencies, where optical signal is propagating through a channel waveguide, the design of the waveguide is almost decoupled from the design of the RF electrodes that help generate the electric field in the LiNbO3 lattice. This decoupling generally means that optical structure, which is a channel waveguide in this case, can almost independently from the RF transmission line electrodes be optimized. This dramatically simplifies the engineering design and optimization efforts.
However, in the case of semiconductor modulators, for instance silicon modulators, the situation is dramatically different. There, the interaction of light and electricity is done through charged particles whose movements are controlled by the applied electric signal. Since the charge transport occurs in the same material system where the optical signal is traveling, the design and optimization of the spatial profile of the optical signal and the electrical port(s) through which the movement of charged particles are controlled, are highly interrelated. Consequently, the engineering effort for design and optimization often requires making delicate tradeoffs among competing performance metrics.
As an example, normally larger carrier concentrations lead to stronger optical effects, whereas through free carrier absorption mechanism, larger concentrations mean larger attenuation of the optical signal. As another manifestation of this interdependency, one can notice that larger doping concentrations mean smaller resistivity inside the semiconductor, henceforth facilitating movement of free carriers, and eventually faster operation speed. On the flip side, larger doping concentrations detrimentally affect the attenuation of RF signals driving the electrical ports, acting as a limiting factor in the speed of operation.
A technique widely used over the past four decades to enhance the operation speed of optical modulators has been velocity matching [1]. The idea is to basically match the speed of propagating optical signal (group velocity of light) with the propagation speed of RF signal using a properly designed transmission line. This technique applied to lithium niobate platform uses modulators that are in the order of 5 cm long and form the workhorse of long-haul optical telecom links. However, the biggest disadvantage of the technique is that, it is not energy efficient.
U.S. Pat. No. 7,039,258 B2 [2] describes an optoelectronic device. This prior art only addresses the issue of speed (bandwidth). This prior art does not have the intention of obtaining energy efficiency or power efficiency.
FIG. 1 is a prior art optical modulator disclosed in U.S. Pat. No. 7,039,258. It is apparent that the electrical signals at the input electrical ports of consecutive modulator elements (M−1 to M−k) experience separate delays by using delay elements (D−1 to D−(K−1)). Therefore, if more modulator elements are used, more delay elements are necessary, which increases both complexity and energy consumption.
As we will explain in details later, the goal of our current invention is simultaneous enhancement of energy efficiency and speed (bandwidth) as two critical metrics of optical modulating systems. Our invention removes all the aforementioned limitations of the prior art by virtue of configuring the apparatus according to a specific architecture, and by doing so enables significant increase in the energy efficiency of the apparatus, while simultaneously achieving extremely high speed (bandwidth).
In U.S. Pat. No. 7,515,778 B2 [26], independent claim 1 states “each drive signal source separately energized/de-energized to control the phase shift”. In the description, column 3 states “For reasons of power dissipation, it is desirable to design a modulator driver that can trade optical extinction ratio for power. Adjusting the output amplitude of most drivers to accomplish this goal is problematic; it usually results in a change in edge rate (i.e., transition time between an optical “1” and optical “0”) and, depending on driver topology, might not reduce power dissipation. An alternative to adjusting the amplitude of the driver to adjust the extinction ratio is to realize that the extinction ratio is based on the phase shift between the two arms of the MZI. Therefore, it has been found that the extinction ratio of the output optical signal can be adjusted (While maintaining a constant electrical swing on the drive voltage) by dynamically adjusting the effective length of the modulated portion of the modulator structure.” Again here, the requirement that all drivers have to energize/de-energize “separately” is the biggest limitation of this prior art, and we will remove this limitation as explained later to significantly reduce energy dissipation.
In the description of U.S. Pat. No. 7,515,778, it is desirable to dynamically adjust the length instead of adjusting the amplitude, and as such indeed their drivers have to be fully separate and working independently. This is a major limitation of this prior art.
In U.S. Pat. No. 8,665,508 [8], US 2014/0,186,028 [9], and U.S. Pat. No. 8,238,014 [11], multi-level PAM modulation at high speed is disclosed. However, we disclose how energy efficient as well as high speed multi-level optical modulation, including but not limited to PAM, can be achieved.
In U.S. Pat. No. 9,111,730, FIG. 13 and column 21 state “By using the high index contrast waveguide technology as described in various embodiments of this invention, it is possible to “fold” the Mach-Zehnder modulator device as shown in FIG. 13. The arms (1301) of the Mach-Zehnder modulator device are shown in FIG. 13. Preferably, the electrodes are placed with alternate voltage in order to achieve modulation. For simplification of the figure, the electrodes are not drawn in FIG. 13. It is possible to achieve traveling-wave RF electrodes in this structure, but the polarity of the wave must be alternated, as shown in FIG. 13, to function as a modulator.”
U.S. Pat. No. 9,111,730 describes how to make a paper-clip structure. It also makes comments about making traveling wave electrodes for this structure. However, traveling-wave electrodes exhibiting sharp bends needed for such folded paper-clip structures experience large losses at high frequencies, in addition to all the disadvantages that have been described before. This prior art again, by virtue of previous explanations, requires high voltage drivers and as such silicon based drivers cannot be used.
The present invention offers a compact, small form-factor, very high speed, highly energy efficient, and extremely low cost solution. The present invention also relaxes the typical high voltage headroom requirements of optical modulators, which also enables the usage of low-cost silicon low-voltage high-frequency drivers.
In U.S. Pat. No. 8,744,219, independent claim 1 states “wherein an i (1≤i≤m, i is an integer)-th individual driving circuit comprises: a driving circuit that outputs a signal obtained by amplifying a digital input signal in synchronization with a clock signal to an i-th waveguide-type optical phase modulator region; and a phase shift circuit that applies a delay to at least a signal branched from the clock signal and outputs the signal, . . . ”
Here again, the prior art describes a structure where there exists an electrical phase shift (electrical delay) element between each consecutive modulating element.
Other references have been cited in the “References” section presented after the detailed description section and before the claims.